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POC to Python via SWIG: can't get void** parameters to hold their value
 

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    <p>I have a C interface that looks like this (simplified):</p> <pre><code>extern bool Operation(void ** ppData); extern float GetFieldValue(void* pData); extern void Cleanup(p); </code></pre> <p>which is used as follows:</p> <pre><code>void * p = NULL; float theAnswer = 0.0f; if (Operation(&amp;p)) { theAnswer = GetFieldValue(p); Cleanup(p); } </code></pre> <p>You'll note that Operation() allocates the buffer p, that GetFieldValue queries p, and that Cleanup frees p. I don't have any control over the C interface -- that code is widely used elsewhere.</p> <p>I'd like to call this code from Python via <a href="http://www.swig.org/" rel="nofollow noreferrer">SWIG</a>, but I was unable to find any good examples of how to pass a pointer to a pointer -- and retrieve its value.</p> <p>I think the correct way to do this is by use of typemaps, so I defined an interface that would automatically dereference p for me on the C side:</p> <pre><code>%typemap(in) void** { $1 = (void**)&amp;($input); } </code></pre> <p>However, I was unable to get the following python code to work:</p> <pre><code>import test p = None theAnswer = 0.0f if test.Operation(p): theAnswer = test.GetFieldValue(p) test.Cleanup(p) </code></pre> <p>After calling test.Operation(), p always kept its initial value of None. </p> <p>Any help with figuring out the correct way to do this in SWIG would be much appreciated. Otherwise, I'm likely to just write a C++ wrapper around the C code that stops Python from having to deal with the pointer. And then wrap <em>that</em> wrapper with SWIG. Somebody stop me!</p> <p>Edit:</p> <p>Thanks to <a href="https://stackoverflow.com/users/7161/jorenko">Jorenko</a>, I now have the following SWIG interface:</p> <pre><code>% module Test %typemap (in,numinputs=0) void** (void *temp) { $1 = &amp;temp; } %typemap (argout) void** { PyObject *obj = PyCObject_FromVoidPtr(*$1, Cleanup); $result = PyTuple_Pack(2, $result, obj); } %{ extern bool Operation(void ** ppData); extern float GetFieldValue(void *p); extern void Cleanup(void *p); %} %inline %{ float gfv(void *p){ return GetFieldValue(p);} %} %typemap (in) void* { if (PyCObject_Check($input)) { $1 = PyCObject_AsVoidPtr($input); } } </code></pre> <p>The python code that uses this SWIG interface is as follows:</p> <pre><code>import test success, p = test.Operation() if success: f = test.GetFieldValue(p) # This doesn't work f = test.gvp(p) # This works! test.Cleanup(p) </code></pre> <p>Oddly, in the python code, test.GetFieldValue(p) returns gibberish, but test.gfv(p) returns the correct value. I've inserting debugging code into the typemap for void*, and both have the same value of p! The call Any ideas about that?</p> <p><strong>Update:</strong> I've decided to use ctypes. MUCH easier.</p>
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    1. CO1. `p` is simultaneously 'void**' and 'void*' in your Python code 2. `None` is immutable therefore `p` (an object it refers to) won't change no matter what (though you may bind `p` to a different object (using `=`)).
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